This invention relates to an apparatus and a method for refurbishing valve seats, and is more particularly concerned with refurbishing valve seats of high temperature, high pressure safety valves in situ.
High temperature, high pressure safety valves are commonly used throughout the steam systems of power generation plants. To maintain proper sealing, the valve seats of these valves must be regularly lapped to remove steam cutting and chattering damage. Once more than xc2xc inch of material has been removed from the valve seat, the entire valve must be removed and replaced. Replacement of the valve involves cutting the worn valve from the pipes to which it is connected and installing a new or reconditioned valve in its place. This is a time-consuming, expensive procedure.
Several methods for in situ valve repair are known in the prior art. For example, U.S. Pat. No. 4,593,445 (Snyder et al.), issued on Jun. 10, 1986, describes an apparatus and method for remotely refurbishing valve seats in the steam chest of a turbine generator. The method described by Snyder comprises grinding away a sealing layer of the worn valve seat, building up the valve seat by deposition of weld metal using a gas tungsten arc welder including a solid wire feeder for feeding chromium-steel wire to the electrode. The next step of the process comprises machining of the buildup of weld metal to achieve the desired profile.
One disadvantage of the Snyder process is that, although, gas tungsten arc welding is believed to provide a higher quality weld than other welding processes, gas tungsten arc welding can also be significantly slower, particularly when producing a weld buildup of a metal such as stainless steel.
The present invention provides a method and apparatus for in situ refurbishment of valve seats in which the valve seat is taken down to a flat surface, a buildup of weld metal is deposited on the flat surface by gas metal arc welding, utilizing a consumable electrode, up to a height and width which is greater than the final dimensions of the valve seat, and the built up valve seat is then machined to its final dimensions.
The use of gas metal arc welding in the process of the present invention, rather than Gas tungsten arc welding as in the Snyder et al. patent, permits more rapid buildup of weld metal on the ground down surface of the valve seat, resulting in improved efficiency. In general, the use of gas metal arc welding permits a wider bead of weld metal to be laid down on the valve seat. In order to assist in maintaining the quality of the buildup, support structures are provided at the inner perimeter and outer perimeter of the valve seat to support the buildup as the weld metal is being laid down and to prevent roll over of the metal beads.
Furthermore, since the buildup of weld metal is laid down more rapidly in the method of the invention, the support structures preferably comprise solid chill blocks made of a metal such as copper or stainless steel which act as heat sinks to dissipate heat away from the buildup as it is being laid down. Preferably, a pair of chill blocks are provided, comprising an inner block in the form of a solid cylinder or a cylindrical sleeve or ring which is mounted in the hollow interior of the valve inward of the valve seat, and an outer chill block comprising two C-shaped rings which clamp around the upper end of the valve nozzle.
The use of support structures such as chill blocks permits a high quality buildup to be maintained through the use of gas metal arc welding.
Accordingly, in one aspect the present invention provides an apparatus for refurbishing a worn valve seat of a high temperature, high pressure safety valve in situ, the valve seat being generally circular and being provided at one end of a hollow, cylindrical valve nozzle which is attached to a valve body of said valve, said valve seat being accessible through an access opening in said valve body located above said valve seat, said apparatus comprising: (a) valve machining means mountable to said valve, said valve machining means having first cutter means extendable into the valve body through the access opening for initially machining the worn valve seat to a flat annular surface having an inner perimeter and an outer perimeter; (b) a gas metal arc welding apparatus mountable to the valve having a welding head with a consumable electrode, the welding head being extendable through the access opening and being rotatable in a circle located substantially parallel to and above the flat surface of the valve seat to deposit a buildup of weld metal from said consumable electrode onto said flat surface; and (c) a pair of metal support structures for supporting the buildup of weld metal as it is being deposited by the gas metal arc welding apparatus, said pair of support structures comprising: (i) an inner support structure which is positioned inwardly of the inner perimeter of the flat surface during deposition of the weld metal buildup and which has an outer, substantially vertical cylindrical surface which forms an inner support wall for the weld metal buildup, said inner support wall having a height sufficient to support the weld metal buildup; and (ii) an outer support structure which surrounds the outer perimeter of the flat surface during deposition of the weld metal buildup and which has an inner, substantially vertical cylindrical surface which forms an outer support wall for the weld metal buildup, said outer support wall being located proximate the outer perimeter of the flat surface and having a height sufficient to support the weld metal buildup; said valve machining means also having second cutter means extendable into the valve body through the access opening for machining the weld metal buildup to a desired final height and profile.
In another aspect, the present invention provides a method for refurbishing a worn valve seat of a high temperature, high pressure safety valve in situ, said valve seat being generally circular and being provided at one end of hollow, cylindrical valve nozzle which is attached to a valve body of said valve, said valve seat being accessible through an access opening in said valve body located above said valve seat, the method comprising: (a) initially machining the worn valve seat to a flat annular surface having an inner perimeter and an outer perimeter using a valve machining means mounted to the valve body and having cutter means extending into the valve body; (b) installing an inner support structure inwardly of the inner perimeter of the flat surface of the valve seat, said inner support structure having an outer, substantially vertical cylindrical surface which is located proximate the inner perimeter of the flat surface and extends above the flat surface to at least a predetermined height; (c) installing an outer support structure outwardly of the outer perimeter of the flat surface of the valve seat, said outer support structure having an inner, substantially vertical cylindrical surface which is located proximate the outer perimeter of the flat surface and extends above the flat surface to at least said predetermined height; (d) depositing a buildup of weld metal on said flat surface between said support structures, using a gas metal arc welding apparatus mounted to the valve and having a welding head with a consumable electrode extending through the access opening into the valve body, the buildup of weld metal being deposited while rotating the welding head in a circle substantially parallel to and above the flat surface of the valve seat so as to deposit a buildup of uniform thickness to approximately the predetermined height; and (e) machining the buildup of weld metal to the final desired height, thickness and profile of the refurbished valve seat.